1. Field of the Invention
The present invention relates to a method for reducing polymer sheeting during the polymerization of alpha-olefins, such as ethylene homopolymers and copolymers (including comonomers with a diene). More particularly, the present invention relates to a method for controlling and/or reducing static charges in a reactor, which charges contribute to sheeting.
2. Description of the Prior Art
Polymers of ethylene are conventionally produced in a fluidized bed reactor. During polymerization a phenomenon known as "sheeting" occurs. By "sheeting" is meant the adherence of fused catalyst and resin particles to the walls of the reactor.
In commercial reactors, sheets can vary widely in size, and are usually about 1/4 to 1/2 inch thick and are from about one to five feet long, with a few even longer. They can have a width of about 3 inches to more than 18 inches. The sheets have a core composed of fused polymer which is oriented in the long direction of the sheets and their surfaces are covered with granular resin which is fused to the core. The edges of the sheets can have a hairy appearance from strands of fused polymer.
It has been found that there exists a strong correlation between polymer sheeting and the presence of excess electrostatic of static charges, either negative or positive, in the reactor during polymerization. This is evidenced by sudden changes in static levels followed closely by deviation in temperatures at the reactor wall. These temperature deviations are either high or low. Low temperatures indicate particle adhesion causing an insulating effect from the bed temperature. High deviations indicate reaction taking place in zones of limited heat transfer. Following this, disruption in fluidization patterns is generally evident, such as, for example, catalyst feed interruption, plugging of the product discharge system, the occurrence of thin fused agglomerates (sheets) in the product.
Oguchi and Tamatani in an article entitled, "Contact Electrification in Inorganic Binary Compounds," J. Electrochem Soc., Solid State Science and Technology, April 1986, p. 841-847, discuss the measurement of the amount of electric charge induced on a wide variety of powder samples by contact with another reference powder using a blow-off method and apparatus.
U.S. Pat. No. 4,855,370 relates to a method for reducing sheeting during polymerization of alpha-olefins in a low pressure fluidized bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts. The method comprises introducing water into the reactor in an amount sufficient to maintain the electrostatic levels at the site of possible sheet formation at levels which avoid sheeting without substantially altering the effectiveness of said catalysts.
U.S. Pat. No. 4,803,251 also relates to a method for reducing sheeting during polymerization of alpha-olefins in a low pressure fluidized bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts. The method comprises determining the electrostatic levels at the site of possible sheet formations in the reactor. If negative electrostatic levels are indicated then a positive charge generating additive selected from the group consisting of an alcohol containing 1 to 7 carbon atoms, oxygen, and nitric oxide is added to the reactor. If positive electrostatic levels are indicated in the reactor, then a negative charge generating chemical additive is added to the reactor. The negative charge generating chemical additive is a ketone containing up to 7 carbon atoms. The positive or negative charge generating chemical additives are added to the reactor as required in an amount sufficient to create and maintain neutral static charges in the reactor.
Unfortunately, the introduction to the reactor of the additives set forth in the above-mentioned patents can adversely interfere with polymerization and/or the catalyst, i.e., they act as poisons to the system. Accordingly, there is an on-going need to provide a method to reduce sheeting and/or control or reduce static charges in the reactor during polymerization.